An access point, a base station, an evolved node B (eNB), a next generation node B (gNB), a transmit/receive point (TRP), a similar component, or any combination of such components may be referred to herein as a network or a network device. Actions and methods described herein as being performed by a network or a network device may be performed by one or more such components. A network or a network device may transmit downlink reference signals for use by a user equipment (UE), a mobile station, or a similar component.
Downlink transmissions are carried out from a specific antenna port, which is known a priori to a UE receiving the transmissions. From a device perspective, there is a one-to-one mapping between the radio channel experienced by a signal transmitted on the downlink and the antenna port from which the signal has been transmitted. Each antenna port can be seen as corresponding to one downlink reference signal. A downlink reference signal may be used by the UE to estimate the channel corresponding to the antenna port for coherent demodulation and to derive detailed channel state information (CSI) related to the antenna port for, e.g., downlink link adaptation and scheduling or beam management.
Among the downlink reference signals used in Long Term Evolution (LTE) systems for the purpose of channel estimation and CSI acquisition are the common reference signal (CRS), the demodulation reference signal (DM-RS), the CSI reference signal (CSI-RS), the multicast-broadcast single-frequency network (MBSFN) reference signal, and the positioning reference signal.
Downlink reference signals in LTE may be classified as cell-specific reference signals or UE-specific reference signals. Cell-specific reference signals are tied to the physical cell identifier (PCID) and are the same for all the UEs in a cell, once configured. UE-specific reference signals are intended for a specific UE and are not necessarily associated with a specific PCID. The CRS is typically cell-specific and transmitted in a wide band across the system bandwidth. The DM-RS is typically UE-specific and self-contained, i.e., transmitted within the time-frequency resources of UE-specific channels, such as a control or data channel (e.g., the physical downlink control channel (PDCCH) or the physical downlink shared channel (PDSCH) in LTE). In some cases, the same reference signal may be cell-specific or UE-specific, depending on the configuration.